Autotransformer



'Sept. 25, 1956 E. c. HILL AUTOTRANSFORMER Filed March 2, 1954 INVENTOR.Ezra C. /7/' BY W /7/s Attorney.

United States Patent AUTOTRANSFORMER Ezra C. Hill, Fort Wayne, Ind.,assignor to General Electric Company, a corporation of New YorkApplication March 2, 1954, Serial No. 413,571

4 Claims. (Cl. 336-220) This invention relates to electricaltransformers and more particularly to high voltage impulse transformers.

High voltage impulse transformers are used in many applications, forexample, in capacitor discharge systems for operating stroboscopic flashequipment for photography. Such transformers conventionally comprise aprimary winding, usually having a single layer of relatively large wirewound over a core, and a secondary winding usually having a number oflayers of relatively fine wire wound over the primary winding. Thesetransformers are frequently auto-transformer connected and thus when thesecondary winding is Wound over the primary, it is necessary to bringout the inner or starting end of the secondary winding in order toconnect it to one end of th primary winding. Since these transformersare relatively small in size, it has been difficult to pull out theinner end of the fine wire secondary winding thus resulting in thedamaging of an unduly large number of coils in the effort to make thisconnection.

It has been found that if the inner or starting end of the fine wiresecondary winding is left unconnected, the voltage induced in thesecondary winding breaks through the insulation separating the twowindings to complete the autotransformer circuit. In an impulsetransformer application, such as in connection with a flash tube, theenergy level in the arc formed between the two windings is sufficientlylow that the insulation is not injured thereby and the elimination ofthe requirement of bringing out the inner end of the secondary windingand connecting it to the primary winding has effected a marked reductionin the overall cost of the device. Further, the elimination of thisconnection is not only effected without any detriment to the performanceof the device, but in fact results in a slight gain in performance.

It is, therefore, an object of this invention to provide an improvedautotransformer wherein one end of the secondary winding is notconnected to the primary winding.

Further objects and advantages of this invention will become apparentand the invention will be better understood by reference to thefollowing description and the accompanying drawing, and the features ofnovelty which characterize this invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

This invention in its broadest aspects provides an autotransformerhaving a primary winding with its two ends being adapted to be connectedto a source of pulsating voltage. A secondary winding is providedadjacent the primary winding and electrical leads are provided forconnecting one end of the secondary winding and one end of the primarywinding across the load. The other end of the secondary winding is leftfree so that the voltage induced therein breaks through the insulationseparating the windings to complete the autotransfonner circuit.

In the drawing:

Fig. 1 is a schematic illustration showing the application of theimproved autotransformer of this invention in connection with a flashtube; and

2,764,745 Patented Sept. 25, 1956 Fig. 2 is a view in perspective,partly broken away, showing the improved autot-ransformer constructionof this invention.

Referring now to Fig. l, the improved autotransformer of this inventionis schematically shown at 1 and comprises a core 2 with a primarywinding 3 and secondary winding 4 arranged thereon. Primary winding 3has its end-s 5 and 6 respectively connected to one terminal oftriggering capacitor 7 and to grounded common bus 8. The other terminalof capacitor 7 is connected to the mid-point 9 between series resistors10 and 11 which are connected across positive line 12 and common bus 8.A switch 13 is connected across resistor 11 and its function will behereinafter described. A main storage capacitor 14 is connected acrosslines 12 and 8 which in turn are adapted to be connected to an externalsource of direct current (not shown). A photofiash tube 15 is providedhaving its cathode 16 connected to grounded bus 8 and its anode 17connected to positive line 12. The triggering electrode 18 of photofiashtube 15 is connected to one end 19 of secondary winding 4 and the otherend 20, which normally would be connected to end 5 of primary winding 3in order to complete an autotransformer circuit, is left free as shown.

In operation, with lines 8 and 12 connected to a source of directcurrent, storage capacitor 14 is charged as is triggering capacitor 7.When switch 13 is closed, triggering capacitor 7 is discharged throughprimary winding 3 of transformer 1 and since the loop circuit formed bythe switch 13, capacitor 7, primary winding 3 and common line 8 is an LCcircuit, an oscillatory discharge will result. This oscillatorydischarge through primary winding 3 induces a voltage in secondarywinding 4. Secondary winding 4 is preferably constructed with many moreturns than primary winding 3 so that the induced open circuit peakvoltage across primary winding 4 is sufficient to break down theinsulation between primary winding 3 and secondary winding 4. An arc isthus formed between end 20 of secondary winding 4 and primary Winding 3completing an autotransformer connection and placing the combinedvoltages of primary winding 3 in secondary winding 4 across cathode 16and triggering electrode 18 of photofiash tube 15. This voltage issuflicient to ionize the tube and will cause main energy capacitor 14 todischarge across anode 17 and cathode 16 of photoilash tube 15. Whilethe improved autotransformer 1 is shown in connection with a capacitordischarge system for a photofiash tube, it will be readily understoodthat this is by way of illustration only and that the improvedautotransformer 1 may be used in other applications.

Referring now to Fig. 2, there is shown a preferred embodiment of thisinvention in which the autotransformer 1 comprises a cylindrical core 2formed of magnetic material, such as sintered material. The primarywinding 3, preferably comprising a single layer of relatively heavy wireis wound over a spool 19 formed of insulating mate-rial and its ends 5and 6 are left exposed. The starting end 20 of the secondary winding 4,which is preferably formed of relatively fine wire is mechanically butnot electrically connected to the outer surface of the primary winding3, as by suitable adhesive tape 21. A layer insulator 22 is theninserted between the end 20 of secondary winding 4 and primary winding 3and the first layer of the secondary winding 4 is then wound. It will beseen that the outer edge 23 of the insulator 22 overlaps the inner edge24 so that only the end 20 of secondary winding 4 extends between thetwo edges to a position adjacent the primary winding 3. The secondaryWinding 4 is preferably formed of a plurality of layers of fine wirewith layer insulators 25 separating each layer. It will thus be seenthat the outer or finishing end 19 of secondany winding 4 is on theouter periphery of the coil and comes out under the outer edge 27 of theouter layer insulation 25. In order to prevent arcing over between thesecondary winding 4 and primary winding 3 at any point except betweenend 20 and the adjacent turn of primary winding 3, secondary winding 4is axially shorter than primary winding 3 and its axial ends 23 and 29are spaced from the axial ends of primary winding 3. Layer insulation 22and 25 on the other hand is substantially the same length as primarywinding 3 and thus extends substantially beyond the axial ends 23 and ofsecondary winding 4-.

In a transformer actually constructed in accordance with this invention,the primary winding 3 consisted of 46 turns of .0126" diameter wirewound over a spool 26 having a A" diameter. The secondary winding 4consisted of 1850 turns of .0031 diameter wire in several layers. Thistransformer was connected in a circuit shown in Fig. 1 and the primarywinding 3 was energized by discharge of a A microfarad capacitor chargedto 160 volts. This produced a 5000 peak voltage across secondary winding4-. This high voltage pulse was sufficient to break through theinsulation on the fine wire of secondary 4 and the insulation on thewire of primary winding 3, the are being formed between end 20 ofsecondary winding 4 and the adjacent turn of primary winding 3. Theenergy level in this are was sufiiciently low and the durationsufficiently short to prevent injury to the transformer.

It will be readily seen that this improved construction eliminates thenecessity for bringing out the end 20 of secondary winding 4 after thecoil has been wound and for connecting this end to the primary winding.This eliminates the cost of making the connection and further eliminatesthe numerous rejected coils which were damaged in the course of makingthis connection. While the transformer of Fig. 2 has been shown with thesecondary winding 4 wound over the primary winding 3, it will be readilyunderstood that the primary winding may be wound over the secondarywinding or that a different core construction may be used with thesecondary winding being arranged adja cut the primary winding. With theend 20 of secondary winding 4 being unconnected, no voltage appearsacross lines 19 and 8 until the are has been formed between end 20 andprimary winding 3. This provides a very peaked, steep wave front and itis well known that photoflash tubes are more sensitive to higherfrequencies, i. e. to steep wave fronts. it is thus seen that theimproved transformer l is not detrimental to the performance of flashtube 15, but on the contrary provides slightly improved performance.

While I have shown and described a particular embodiment of thisinvention, further modifications and improvements will occur to thoseskilled in the art. I desire that it be understood therefore that thisinvention is not limited to the particular form shown and I intend inthe appended claims to cover all modifications which do not depart fromthe spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An autotransformer comprising a cylindrical core, a primary windinghaving a single layer of wire wound on said core, a sheet of insulationmaterial surrounding said primary winding and having one axial edgeoverlapping its other axial edge, a secondary winding having a pluralityof layers of wire wound over said insulation and havin one endpositioned under said overlapping edge of said insulation and securedadjacent and electrically insulated from said primary winding adjacentone end thereof, said sheet being of a quality to insulate said windingsfrom each other during operation of said auto-- transformer, theinsulation between said primary winding and said one end of saidsecondary winding being of a quality to permit arcing therebetweenduring operation of said autotransformer, said secondary winding beingaxially shorter than said primary winding and its axial edges beingrespectively spaced from the axial edges of said primary winding,insulation separating each of said secondary winding layers, saidinsulation having substantially the same axial length as said primarywinding, and electrical leads connected to the other end of said primarywinding and the other end of said secondary winding respectively wherebysaid windings may be connected across a load.

2. An autotransformer comprising a core, a primary winding wound on saidcore, a secondary winding having a plurality of layers of wire woundover said primary winding with the starting end adjacent said primarywinding and the finishing end on the outer periphery of said secondarywinding, insulating material spacing said secondary winding from saidprimary winding and of a quality to insulate said windings from eachother during operation of said autotransformer, said starting end ofsaid secondary winding extending inwardly past said insulating materialand being secured adjacent and electrically insulated from said primarywinding adjacent an end thereof, the insulation between said primarywinding and said secondary winding starting end being of a quality topermit arcing therebetween during operation of said autotransformer,said secondary winding being axially shorter than said primary windingand its axial edges being respectively spaced from the axial edges ofsaid primary winding, insulation separating each of said secondarywinding layers, and electrical leads connected respectively to the otherend of said primary winding and to said finishing end of said secondarywinding whereby said windings may be connected across a load.

3. An autotransformer comprising a core, a primary winding wound on saidcore, a sheet of insulation material surrounding said primary windingand having one axial edge over-lapping its other edge, a secondarywinding having a plurality of layers of wire wound over said insulationand having one end positioned under said overlapping edge of saidinsulation and secured adjacent and electrically insulated from saidprimary winding adjacent an end thereof, said sheet being of a qualityto in sulate said windings from each other during operation of saidautotransformer, the insulation between said primary winding and saidone end of said secondary winding being of a quality to permit arcingtherebetween during operation of said autotransformer, said secondarywinding being axially shorter than said primary winding and its axialedges being respectively spaced from the axial edges of said primarywinding, insulation separating each of said secondary winding layers,and electrical leads connected respectively to the other end of saidprimary winding and the other end of said secondary winding whereby saidwindings may be connected across a load.

4. An autotransformer comprising a core, a primary winding having asingle layer of wire wound on said core, a secondary winding having aplurality of layers of wire wound over said primary winding with thestarting end adjacent said primary winding and the finishing end on theouter periphery of said secondary winding, insulating material spacingsaid secondary winding from said primary winding and of a quality toinsulate said windings from each other during operation of saidautotransformer, said starting end of said secondary winding extendinginwardly past said insulating material and being secured adjacent andelectrically insulated from said primary winding adjacent an endthereof, the insulation between said primary winding and said secondarywinding starting and being of a quality to permit arcing therebetweenduring operation of said autotransformcr, said secondary winding beingaxially shorter than said primary winding and its axial edges beingrespectively spaced from the axial edges of said primary winding,insulation separating each of said secondary winding layers, saidinsulation having substantially the same axial length as said primarywinding, and electrical leads connected respectively to the other end ofsaid primary winding and to said finishing end of said secondary Windingwhereby said windings may be connected across a load.

References Cited in the file of this patent UNITED STATES PATENTS1,303,162 Cavanagh May 6, 1919 6 Kent Nov. 20, 1 923 Persons Ian. 11,1927 Rietmann Sept. 14, 1937 Stephens Aug. 7, 1945 Crump Apr. 22, 1947FOREIGN PATENTS Great Britain Nov. 23, 1933

